As an exhaust gas purification system for purifying NOx in exhaust gas of a diesel engine, an SCR system using a selective catalytic reduction apparatus (hereinafter, referred to as an SCR apparatus) has been developed.
The SCR system supplies the urea aqueous solution upstream of the exhaust gas of the SCR apparatus, generates ammonia by heat of the exhaust gas, and reduces and purifies NOx on an SCR catalyst by the ammonia (see, for example, Patent Document 1).
In the SCR system, injection of the urea aqueous solution is controlled according to the concentration of NOx in the exhaust gas. A NOx sensor is provided on an exhaust pipe for the above control.
When a detection value of the NOx sensor is not appropriate, the injection of the urea aqueous solution cannot be appropriately controlled, and thus the SCR system includes a NOx sensor diagnosis apparatus that diagnoses the appropriateness of the NOx sensor.
The NOx sensor diagnosis apparatus in the related art is configured to diagnose that the NOx sensor is erroneous when a difference between a calculation value of the concentration of NOx determined according to an actuation state of an engine and the detection value of the concentration of NOx detected by the NOx sensor is larger than a predetermined threshold value (see, for example, Patent Document 2).